1. Technical Field
The invention relates to a liquid crystal compound, a liquid crystal composition and a liquid crystal display device. More specifically, it relates to a four-ring compound having a plurality of CF2O bonding groups, a composition including this compound and having a nematic phase, and a liquid crystal display device containing this composition.
2. Technical Background
A liquid crystal display device is widely used for the display of personal computers, televisions and so forth. This device utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal compound. The operating modes of this liquid crystal display device are known, such as a PC (phase change) mode, a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a BTN (bistable twisted nematic) mode, an ECB (electrically controlled birefringence) mode, an OCB (optically compensated bend) mode, an IPS (in-plane switching) mode, a VA (vertical alignment) mode and a PSA (polymer sustained alignment).
A liquid crystal composition having suitable physical properties has been used for such a liquid crystal display device. It is desirable that a liquid crystal compound included in this composition should have physical properties shown in the following items (1) to (8), in order to further improve the characteristics of the liquid crystal display device.                (1) high stability to heat, light or the like,        (2) high clearing point,        (3) low minimum temperature of a liquid crystal phase,        (4) small viscosity (η),        (5) suitable optical anisotropy (Δn),        (6) large dielectric anisotropy (Δ∈),        (7) suitable elastic constant (K), and        (8) excellent compatibility with other liquid crystal compounds.        
The effect of the physical properties of the liquid crystal compound on the characteristics of the device is as follows. A compound having a high stability to heat, light or the like, as described item (1), increases the voltage holding ratio of the device, as a result of which the service life of the device is increased. A compound having a high clearing point, as described in items (2), increases the temperature range in which the device can be used. A compound having a low minimum temperature of a liquid crystal phase such as a nematic phase and a smectic phase, especially of a nematic phase, as described in item (3), also increases the temperature range in which the device can be used. A compound having a small viscosity, as described in item (4), decreases the response time of the device.
A compound having a suitable optical anisotropy, as described in item (5), improves the contrast of the device. A compound having a large optical anisotropy or a small optical anisotropy, that is to say a suitable optical anisotropy, is necessary according to the design of the device. A compound having a large optical anisotropy is suitable when the response time is decreased by decreasing the cell gap of the device. A compound having a large dielectric anisotropy, as described in item (6), decreases the threshold voltage of the device, as a result of which the electric power consumption of the device is decreased.
In regard to the item (7), a compound having a large elastic constant decreases the response time of the device. A compound having a small elastic constant decreases the threshold voltage of the device. Thus, a suitable elastic constant is necessary according to the characteristics that should be improved. A compound having an excellent compatibility with other liquid crystal compounds is desirable as described in item (8). This is because the physical properties of the composition are adjusted by mixing liquid crystal compounds having different physical properties.
A variety of liquid crystal compounds having a large dielectric anisotropy have been prepared until now. This is because excellent physical properties which are not possessed by conventional compounds are expected. This is because a new compound is expected to possess a suitable balance between two physical properties which are required for the preparation of a liquid crystal composition. Compounds having CF2O bonding group is described in Patent documents Nos. 1 to 4. However, each compound does not have a sufficiently large dielectric anisotropy, and thus a liquid crystal composition including this compound does not seem to satisfy threshold voltage that is necessary for commercial devices.

A three-ring compound having two CF2O bonding groups, the compound (S-1), is shown in Patent document No. 5. However, this compound does not have a sufficiently high clearing point. Five-ring compounds having two CF2O bonding groups, the compounds (S-2) and (S-3), are shown in Patent documents Nos. 6 and 7. However, these compounds do not have a sufficiently small viscosity.
In view of these situations, a compound with excellent physical properties and a suitable balance concerning the physical properties (1) to (8) described above has been expected to be developed.